As the known apparatus for splicing spun yarns, there can be mentioned fisherman's knot as shown in FIG. 1a and weaver's knot as shown in FIG. 1b. In these knotters, the knotting operation heretofore conducted manually by a worker is performed only mechanically and the structure of the formed knot is not different from that of the knot manually formed by the worker. Such fisherman's or weaver's knot can comply with mass production by mechanization, but since increase of the knot strength alone is mainly intended in such yarn-tying apparatus, the size of the knot becomes as large as about 3 times the diameter of the single yarn, and any particular consideration is not paid to this increased size of the knot. This has serious influences on the subsequent processing steps. For example, the size of the knot which is about 3 times the diameter of the single yarn causes breakage on knitting needles at the knitting step, inhibiting continuous operation of the machine, and a perforated knitted fabric is readily formed. Furthermore, in an air or water jet loom, the yarn end projecting from a weft falls in contact with a warp forming a shed and there occurs an undesirable phenomenon in which the weft does not arrive at the fabric end. Moreover, knots appearing on the woven fabric as the final product are regarded as defects and it is necessary to perform a post treatment of removing a knot-appearing portion from the woven fabric or pushing the knots toward the back side of the fabric.
As means for eliminating the foregoing defects, there has been proposed a yarn-splicing apparatus which provides a knot structure quite different from the knot structure produced by the fisherman's knotter or weaver's knotter. According to this proposal, a fluid is jetted on lapped yarn ends to mingle the yarn ends and entangle fibers of the yarn ends to effect knotting. The knot structure thus produced is in principle as shown in FIG. 1c. After the yarn ends have been mingled with each other, the fibers of both the yarn ends are entangled with one another and certain twists are given to the fibers, whereby an integrated knot structure is formed. Accordingly, the knot produced by such air knotter comprises entangled portions f1 and f2 present in the vicinity of the yarn ends and a twisted portion f3 located between the entangled portions f1 and f2.
As pointed out hereinbefore, the size of the knot formed by the fisherman's or weaver's knotter is at least about 3 times the diameter of the single yarn. In contrast, the size of the knot formed by the air knotter is not larger than about 1.5 times the diameter of the single yarn. Supposing that the ends of a single yarn having a diameter d1 and a sectional area A1, are spliced by the air knotter to form a knot having a diameter d2 and a sectional area A2, since A1 is equal to .pi./4 d1.sup.2 and A2 is equal to the sum of the sectional areas of the two single yarns, A2 is expressed as follows: EQU A2=A1+A1=(.pi./4) d1.sup.2 +(.pi./4) d1.sup.2 ( 1) EQU A2=(.pi./4) d2.sup.2 ( 2)
From the formula (1) and (2), the following relation is derived: ##EQU1## Accordingly, the size d2 of the knot is .sqroot.2 times the size f1.
The above value is a theoretical value calculated based on the supposition that the two single yarns d1 are completely mingled with each other and the knot has a shape of a true circle. It is estimated that practically, the knot is elliptical more or less. Accordingly, it is estimated that the maximum size of the knot is somewhat larger than .sqroot.2 times the single yarn diameter d1. However, at any rate, this size is about 1/2 of the size of the fisherman's or weaver's knot, which is about 3 times the single yarn diameter. Therefore, this knotting method can be regarded as an epoch-making yarn tying method.
In connection with the binding strength of the knot formed by tying yarn ends, it is preferred that the strength of the knot be equal to or higher than the strength of the single yarn.
In case of the fisherman's or weaver's knot, it is considered that in principle, the binding strength is equal to or higher than the single yarn strength, though in certain yarns, it happens that the binding strength is lower than the single yarn strength. However, in each of knots formed by various air knotters, the binding strength is lower than the single yarn strength, though the binding strength differs depending on the kind of the yarn, for example, a polyester/cotton mix-spun yarn or an acrylic yarn or on the count number of the yarn. Influences of the count number of the yarn on the binding strength of the knot are especially prominent. More specifically, as the size of the yarn is small, the ratio of the binding strength of the knot to the single yarn strength is relatively high, and a value of 70 to 85% was obtained at experiments.
On the other hand, as the size of the yarn is increased, the above-mentioned ratio is decreased and it often happens that the binding strength is less than 50% of the single yarn strength. Furthermore, it is pointed out that even if the count number is the same, the binding strength varies according to the condition of the knot and the stability of the knot is reduced if the knot condition is bad.